Some applications require steel parts combining high mechanical strength, high impact resistance and good corrosion resistance. This type of combination is particularly desirable in the automotive industry which requires a significant reduction in vehicle weight and excellent capacity to absorb energy in the event of a collision. This can be achieved in particular by using steel with very good mechanical characteristics having a martensitic or bainitic-martensitic microstructure: anti-intrusion, structural or safety components of automotive vehicles such as bumpers, door reinforcements, B-pillar reinforcements or roof reinforcements, for example, require the above qualities.
Patent EP 0971044 discloses a fabrication method in which hot- or cold-rolled steel plate coated with aluminum of aluminum alloy is the starting material. After shaping to produce a part, and before heat treatment at a temperature above Ac1, the coating is heated to form a surface alloy by interdiffusion between the steel and the aluminum coating. This alloy prevents decarburization of the metal and oxidation during heat treatment in a furnace. It therefore eliminates the necessity for furnaces containing a special atmosphere. The presence of this alloy also obviates certain surface operations on the treated parts, such as shot blasting, which operations are necessary for plates having no coating. The parts are then cooled under conditions adapted to confer a tensile strength that can exceed 1500 MPa.
With the aim of reducing vehicle weights, parts have been developed consisting of steel blanks of different compositions or different thicknesses continuously butt-welded together. These welded parts are known as “butt-welded blanks”. Laser beam welding is a preferred method of assembling such blanks, exploiting the flexibility, quality and productivity characteristics of the process. After these welded blanks have been cold-pressed, parts are obtained having mechanical strength, pressability, impact absorption properties that vary within the parts themselves. It is therefore possible to provide the required properties at the appropriate location without imposing an unnecessary or costly penalty on all of the parts.
The fabrication method described in patent EP 0971044 can be applied to butt-welded blanks in the following manner: starting from steel plate, possibly of different compositions or thicknesses, and having a metal pre-coating, butt-welded blanks are obtained by a welding process. These welded blanks then undergo heat treatment to form a surface alloy and are then hot-pressed and quenched. This produces quenched parts with thicknesses and intrinsic mechanical characteristics that vary and represent an ideal response to local loading requirements.